# Arithmetic Expressions, Variables, and Functions

## Basic Usage

In critic2, an arithmetic expression can be used almost everywhere in the input where a real or integer number is expected. Arithmetic expressions that appear in the input (without an associated keyword) are evaluated and their result is written to the output. For instance, you can start critic2 and write:

3+2*sin(pi/4)
%% 3+2*sin(pi/4)
4.4142135623731


Similarly, variables can be defined and utilized in any expression. Variable names must start with a letter and are composed only of letters, numbers, and the underscore character. Also, they cannot have the same name as a known constant (pi, e, eps) or a function, regardless of case. Variables in expressions are case-sensitive. To use a variable, first you need to assign it. For instance:

a = 20+10
a/7 + 1
%% a/7 + 1
5.2857142857143


In fact, by using the -q command-line option, critic2 can be used as a simple calculator:

$echo "1+erf($RANDOM/100000)" | critic2 -q
1.2578249310340


When used in combination with other keywords, arithmetic expressions must be enclosed in either double quotes (“), single quotes (‘), or parentheses, or they must form a single word (i.e. no spaces). For instance, this is valid critic2 input:

a = 0.12
[...]
Be 1/3 "2 / 3" 1/4+a


but this is not:

Be 1/3 2 /3 1/4+a


Arithmetic expressions can contain:

• Operators: +, -, *, /, ** or ^, % (modulo)

• Relational and binary operators (return 1 if true and 0 if false): <, >, ==, <=, >=, !=, &&, ||.

• Functions: the usual mathematical functions (sin, exp, etc.) as well as “chemical functions”.

• Constants: pi, e and eps (the machine precision).

• Variables defined by the user as above.

• Structural variables

Parentheses can be used and the usual rules of associativity and precedence apply.

In some cases, arithmetic expressions can be applied to make new scalar fields by transforming existing fields. Scalar fields are denoted by a dollar sign ($) followed by an identifier and, optionally, a modifier separated by a colon ($id:modifier). The default identifier for a field is the order in which the field was loaded in the input. For instance, if the first field ($1) is the spin-up density and the second field ($2) is the spin-down density, the total density can be calculated with the expression $1+$2 and the spin density can be calculated with $1-$2. Field number zero ($0 or $rho0) represent the promolecular density, which is always available once the crystal or molecule structure is known. In our example, $1+$2-$rho0 would represent the density difference between the actual density and the sum of atomic densities. Fields can also be referred by a name, if the ID keyword is used. Named fields simplify work when you have multiple fields. It is possible to specify a field modifier right after the number or name for that field in order to access its derivatives and other properties related to it. The modifier is separated by the field name using the “:” character and is case insensitive. It may be one of: • v: valence-only value of the field (it is usually employed to access the valence density in a grid field in which core augmentation is active). • c: core-only value of the field. • x, y, z: first derivatives: $$f_x$$, $$f_y$$, $$f_z$$. • xx, xy, yx, xz, zx, yy, yz, zy, zz: second derivatives: $$f_{xx}$$, $$f_{xy}$$, etc. • g: norm of the gradient, $$\lvert\nabla f\rvert$$. • l: Laplacian, $$\nabla^2 f$$. • lv: valence Laplacian (Laplacian without core augmentation). • lc: core Laplacian. For instance, $2:l is the Laplacian of field 2 and $rho0:xy is the xy-component of the Hessian of the promolecular density. In molecular wavefunctions (wfn/wfx/fchk/molden), the value of particular orbitals can be selected with the following field modifiers: • an integer (<n>): selects molecular orbital number n. For instance, $1:3 refers to the value of molecular orbital number 3 in field 1.
• HOMO: the highest-occupied MO in an RHF wavefunction.
• LUMO: the lowest-unoccupied MO in an RHF wavefunction.
• AHOMO: the alpha highest-occupied MO in a UHF wavefunction.
• ALUMO: the alpha lowest-unoccupied MO in a UHF wavefunction.
• BHOMO: the beta highest-occupied MO in a UHF wavefunction.
• BLUMO: the beta lowest-unoccupied MO in a UHF wavefunction.
• A<n>: alpha MO number n in a UHF wavefunction.
• B<n>: beta MO number n in a UHF wavefunction.

Using these flags to select virtual orbitals requires a file format that contains information about them (wfn and wfx do not work). In addition, the molecular wavefunction field has to be loaded using the READVIRTUAL keyword. Also, in unrestricted molecular wavefunction fields, one can access the spin densities:

• up: up-spin density.
• dn: down-spin density.
• sp: (difference) spin density.

In critic2, there is a distinction between expressions that reference fields and those that do not and, for certain keywords, critic2 will decide what to do with an expression based on this distinction. For instance:

a = 2

cube cell field "2 * $ewald"  calculates a grid using 2 times the value of the Ewald potential. ## List of Available Functions Arithmetic expressions can use any of the functions in the critic2 function library. The list of functions includes the usual mathematical functions (like exp or sin) but also functions that are meant to be applied to scalar fields of a certain type (e.g., the Thomas-Fermi kinetic energy density, gtf). The latter are called “chemical functions”, since they carry chemical information. It is very important to distinguish whether a function expects a numerical argument (e.g. sin(x)), or a field identifier (e.g. gtf(1) or gtf(rho0)). The list of arithmetic functions is: abs, exp, sqrt, floor, ceil, ceiling, round, log, log10, sin, asin, cos, acos, tan, atan, atan2, sinh, cosh, erf, erfc, min, max. All these functions apply to numbers (or other arithmetic expressions), and their behavior is the usual. For instance, sin(2*pi), max($1,0), and atan2(y,x) are all valid expressions.

The chemical functions in the lists below accept one or more field identifiers as their arguments. Their purpose is to provide shorthands to build fields from other fields using physically-relevant formulas. For instance, gtf(1) is the Thomas-Fermi kinetic energy density calculated using the electron density in field 1. In all instances, gtf(1) is equivalent to writing the formula in full (3/10*(3*pi^2)^(2d0/3d0)*$1^(5/3)) but, naturally, much more convenient. Some of the chemical functions like, for instance, those that require having access to the one-electron wavefunctions (e.g. the ELF), can only be used with fields of a certain type. The name in square brackets, if available, is a shorthand for applying the chemical function to the reference field (case-insensitive) in the POINTPROP keyword. The following list of chemical functions can be used with any field type. In all cases, field id should correspond to the system’s electron density (it is up to the user to make sure this is the case). • gtf(id) [GTF]: Thomas-Fermi kinetic energy density. The kinetic energy density for a uniform electron gas with its density given by the value of field id at the point ($$g^{\text{TF}}$$).1 • vtf(id) [VTF]: the potential energy density calculated using the Thomas-Fermi kinetic energy density and the local virial theorem ($$v^{\text{TF}}({\bf r}) = \frac{1}{4}\nabla^2\rho({\bf r}) - 2g^{\text{TF}}({\bf r})$$ in au).1 • htf(id) [HTF]: the total energy density calculated using the Thomas-Fermi kinetic energy density and the local virial theorem ($$h^{\text{TF}}({\bf r}) = g^{\text{TF}}({\bf r}) + v^{\text{TF}}({\bf r})$$). The field id must contain the electron density of the system.1 • gtf_kir(id) [GTF_KIR]: Thomas-Fermi kinetic energy density with the semiclassical gradient correction proposed by Kirzhnits for the not-so-homogeneous electron gas. The electron density and its derivatives are those of field id at every point in space.($$g^{\text{kir}}$$) 23456 • vtf_kir(id) [VTF_KIR]: the potential energy density calculated using gtf_kir(id) and the local virial theorem ($$v^{\text{kir}}({\bf r}) = \frac{1}{4}\nabla^2\rho({\bf r}) - 2g^{\text{kir}}({\bf r})$$ in au). 23456 • htf_kir(id) [HTF_KIR]: the total energy density calculated using gtf_kir(id) and the local virial theorem ($$h^{\text{kir}}({\bf r}) = g^{\text{kir}}({\bf r}) + v^{\text{kir}}({\bf r})$$). 23456 • lag(id) [LAG]: the Lagrangian density ($$-\frac{1}{4}\nabla^2\rho$$). • lol_kir(id) [LOL_KIR]: the localized-orbital locator (LOL) with the kinetic energy density calculated using the Thomas-Fermi approximation with Kirzhnits gradient correction.7 • rdg(id) [RDG]: the reduced density gradient, \begin{equation} s({\bf r}) = \frac{\lvert{\bf \nabla}\rho({\bf r})\rvert}{2(3\pi^2)^{1/3}\rho({\bf r})^{4/3}} \end{equation} The following functions require the kinetic energy density, and therefore can only be used with fields that provide the one-electron wavefunctions. At present, this is only available for molecular wavefunction fields. • gkin(id) [GKIN]: the kinetic energy density, G-version ($$\sum_i{\bf \nabla}\psi_i\cdot{\bf \nabla}\psi_i /2$$).89 • kkin(id) [KKIN]: the kinetic energy density, K-version ($$\sum_i\psi_i\nabla^2\psi_i /2$$).89 • vir(id) [VIR]: the electronic potential energy density, also called the virial field.10 • he(id) [HE]: the electronic energy density, vir(id) + gkin(id). • elf(id) [ELF]: the electron localization function (ELF).11 • lol(id) [LOL]: the localized-orbital locator (LOL).1213 • brhole_a1(id), brhole_a2(id), brhole_a(id): the $$A$$ prefactor of the spherically averaged hole model proposd by Becke and Roussel (spin up, down, and average, respectively). The BR hole is an exponential $$Ae^{-\alpha r}$$ at a distance $$b$$ from the reference point.14 • brhole_b1(id), brhole_b2(id), brhole_b(id): the $$b$$ parameter of the BR hole model (spin up, down, and average). $$b$$ is distance from the exponential center to the reference point.14 • brhole_alf1(id), brhole_alf2(id), brhole_alf(id): the exponent of the BR hole model (spin up, down, and average).14 • xhcurv1(id), xhcurv2(id), xhcurv(id): the curvature of the exchange hole at the reference point (spin up, down, and average). $$Q_{\sigma}$$ in the literature.14 • dsigs1(id), dsigs2(id), dsigs(id): the leading coefficient of the same-spin pair density (spin up, down, and average). $$D_\sigma$$ in the literature.14 The following chemical functions require both a molecular wavefunction and basis set information (at present, this can only be read from a Gaussian fchk file). In addition, it is necessary to have critic2 compiled with the libcint library to calculate the molecular integrals involved. • mep(id): molecular electrostatic potential. • uslater(id): Slater potential $$U_x$$. The HF exchange energy is $$\int\rho({\bf r})U_x({\bf r})d{\bf r}$$.15 • nheff(id): reverse BR efefctive hole normalization.15 • xhole(id,x,y,z): Exchange hole with reference point at (x,y,z). The coordinates are Cartesian in angstrom referred to the molecular origin if the system is a molecule or crystallographic coordinates if the system is a crystal. Other special labels can be used, that activate the calculation of properties for the reference field. These are: • stress: calculate the Schrodinger stress tensor of the reference field. The virial field is the trace of this tensor.10 A particular case of chemical function is xc(), that allows the user to access the libxc library. This is only possible if the libxc library was linked in the compilation of critic2. ## Use of LIBXC in Arithmetic Expressions If critic2 is linked to the libxc library then the xc() chemical function can be used in arithmetic expressions. xc() calculates the exchange and/or correlation energy density for one of the functionals in the libxc library. The number of arguments to xc() depends on the type of functional invoked, which is selected using an integer index. The list of functionals available and their corresponding indices should be consulted in the libxc documentation. The integer index that selects the functional always appears last in the calling sequence of xc(). The number and type of arguments to xc(...,idx) depend on the type of functional specified by the idx integer. This integer depends on the libxc version used but a full list can be obtained within critic2 using the LIBXC keyword. The call to xc() can be one of: • LDA functional: xc(rho,idx) • GGA functional: xc(rho,grad,idx) • meta-GGA functional: xc(rho,grad,lapl,tau,idx) where rho is the electron density ($$\rho({\bf r})$$), grad is its gradient ($$\lvert\nabla\rho({\bf r})\rvert$$), lapl is its Laplacian ($$\nabla^2\rho({\bf r})$$) and tau is the kinetic energy density ($$1/2\sum_i\lvert\nabla\psi_i\rvert^2$$). Note that rho, grad, lapl, and tau are expressions, not field identifiers as in the chemical functions above or in the idx argument. For instance, the expression for LDA using the electron density loaded in field number 1 would be: xc($1,1)+xc($1,9)  because idx=1 is Slater’s exchange and idx=9 is Perdew-Zunger correlation. PBE (a GGA functional) would be: xc($1,$2,101)+xc($1,$2,130)  Here idx=101 is PBE exchange and idx=130 is PBE correlation. Field $1 contains the electron density and $2 is its gradient. In fields given on a grid spanning the crysatl unit cell, the gradient field can be calculated using: LOAD AS GRAD 1  which is calculated using fast Fourier transform. Alternatively, if the field is not a grid, the gradient is best calculated directly using the :g field modifier: xc($1,$1:g,101)+xc($1,$1:g,130)  ### List Available LIBXC Functionals (LIBXC) When critic2 is compiled with libxc support, the LIBXC keyword can be used to query the library for a list of available functoinals: LIBXC [REF|REFS] [NAME|NAMES] [FLAGS] [ALL]  By default, the LIBXC keyword gives a list of the functional IDs in the first column, followed by the functional name, kind, and family. If FLAGS is used, then flags containing additional information about the functional (e.g. if it provides the exchange-correlation energy, etc.) are given. If NAME or NAMES is used, the full name of the functional is printed. If REF or REFS is used, the literature references are also printed. The ALL option is equivalent to using REFS, NAMES, and FLAGS all at once. ## List of Structural Variables When creating new fields as transformations of existing fields, it is possible to use the crystal or molecular structure (the nearest atom, the coordinates, etc.) as part of the transformation using structural variables. Structural variables start with the symbol “@” followed by an identifier that selects the type of variable. For some of the structural variables, an additional modifier can be applied using the “:” symbol after the variable identifier. For instance, @dnuc gives the distance to the nearest nucleus. @rho0nuc:3 is the atomic density at the given point of the nearest atom number 3 (from the complete list). The following structural variables are accepted in critic2: • dnuc: Distance to the closest nucleus. By default, the distance is in angstrom for molecules and in bohr for crystals, unless changed using the UNITS keyword.16 • xnucx, ynucx, znucx: x,y,z coordinates of the nearest nucleus in crystallographic coordinates.16 • xnucc, ynucc, znucc: x,y,z coordinates of the nearest nucleus in Cartesian coordinates. By default, the coordinates have units of bohr in crystals, and are referred to the molecular center and have units of angstrom in molecules.16 • xx, yx, zx: the x,y,z coordinates of the point where the arithmetic expression is being evaluated (crystallographic coordinates). • xc, yc, zc: the x,y,z Cartesian coordinates of the point where the arithmetic expression is being evaluated. Units are bohr. • xm, ym, zm: the x,y,z Cartesian coordinates of the point where the arithmetic expression is being evaluated. By default, the coordinates have units of bohr in crystals, and are referred to the molecular center and have units of angstrom in molecules. • xxr, yxr, zxr: the x,y,z coordinates of the point where the arithmetic expression is being evaluated (crystallographic coordinates in the reduced unit cell). • x, y, z: the x,y,z coordinates of the point where the arithmetic expression is being evaluated. If the system is a crystal, they are fractional coordinates (same as xx, yx, zx). If the system is a molecule, they are Cartesian coordinates referred to the molecular center and with units of angstrom (same as xm, ym, zm). This is often the most intuitive choice of coordinates. • idnuc: complete-list ID of the closest nucleus.16 • nidnuc: non-equivalent list ID of the closest nucleus.16 • rho0nuc: atomic density contribution from the nearest nucleus.16 • spcnuc: species ID of the nearest nucleus.16 • zatnuc: atomic number of the closest nucleus.16 For instance, if we have a crystal structure and its electron density is grid field $1, then:

LOAD AS "(@idnuc == 1) * $1" ID voronoi SUM VORONOI  calculates the Voronoi charge of atom 1. Likewise, LOAD AS "@rho0nuc:1/$0 * $1" ID hirsh SUM hirsh  calculates the Hirshfeld charge (although it is probably more convenient to use the HIRSHFELD keyword. Structural variables are also useful in molecules in combination with the MOLCALC keyword. For instance, to calculate the dipole moment of a neutral molecule (in units of electrons*angstrom): molcalc "$wfx * @xc"
molcalc "$wfx * @yc" molcalc "$wfx * @zc"


Similar expressions can also be used to create new scalar fields by restricting or modifying the values of a scalar field only in certain areas of the system.

1. Yang and Parr, Density-Functional Theory of Atoms and Molecules.  2 3

2. Kirzhnits, (1957). Sov. Phys. JETP, 5, 64-72.  2 3

3. Kirzhnits, Field Theoretical Methods in Many-body Systems (Pergamon, New York, 1967).  2 3

4. Abramov, Y. A. Acta Cryst. A (1997) 264-272.  2 3

5. Zhurova and Tsirelson, Acta Cryst. B (2002) 58, 567-575.  2 3

6. Espinosa et al., Chem. Phys. Lett. 285 (1998) 170-173.  2 3

7. Tsirelson and Stash, Acta Cryst. (2002) B58, 780.

8. Bader and Beddall, J. Chem. Phys. (1972) 56, 3320.  2

9. Bader and Essen, J. Chem. Phys. (1984) 80, 1943.  2

10. Keith et al. Int. J. Quantum Chem. (1996) 57, 183-198.  2

11. Becke and Edgecombe J. Chem. Phys. (1990) 92, 5397-5403.

12. Schmider and Becke, J. Mol. Struct. (Theochem) (2000) 527, 51-61.

13. Schmider and Becke, J. Chem. Phys. (2002) 116, 3184-3193.

14. A.D. Becke and M.R. Roussel, Phys. Rev. A 39 (1989) 3761.  2 3 4 5

15. A.D. Becke, J. Chem. Phys. 138 (2013) 074109.  2

16. This structural variables accepts a modifier. A modifier is a colon (:) followed by a number id.i. If given, the modifier restricts the structural variable to atoms with integer ID id.i (from the complete list).  2 3 4 5 6 7 8

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